This patent application is related to copending application Ser. No. A-43795 for Acoustic Sensor for Rotary Machinery filed concurrently herewith.
This invention relates generally to rotating electrical machinery, and more particularly the invention relates to measurement of the air gap between the rotar and stator of such machinery.
Utilities operating large hydroelectric generators have been and continue to be concerned with the dynamic behavior of the air gaps in the machines. Thermal expansions and contraction coupled with centrifugal and magnetic forces produce movement within the mechanical structure of the generator leading to changes in the air gap dimensions. In a worse case situation, the air gap can actually close leading to rotor-stator contact damage. Even when closure does not occur, azimuthal variations in air gap result in comparable variations in air gap magnetic reluctance. As a consequence, there are related variations in magnetic flux coupling of the armature and thus variations in both induced voltage (which is a linear function of flux) and the magnetic force (which is a function of the square of flux density). Since such systems are invariably connected to a low impedance power grid, variations in induced voltage from terminal voltage are compensated by variations in coil currents, known as split phase currents, which in turn cause variations in dissipation and temperature rise.
Attempts have been made to measure dynamic air gap behavior. One such attempt which has provided to be relatively successful in disclosed by Talas and Toom. "Dynamic Measurement and Analysis of Air Gap Variations in Large Hydroelectric Generators", IEE Transactions for Power Apparatus and Systems, Vol. PAS-102, N.9, Sept. 1983, pages 3098-3106. In this systems two bundles of optical fibers are routed from rotor hub to rim where the fiber ends are intermixed and/or oriented so as to be colinear with machine axis and in view of a strip of glass-beaded, retro-reflectors. As the rotor spins, an infrared beam issuing from the fibers of one bundle, reflecting and entering the fibers of the second bundle, is periodically and partially obscured by fins protruding from cooling slots in the stator. A change in the fraction of the fiber obscured by the partial blockage of each fin is a measure of the change in air gap dimensions.
The present invention is directed to an improved apparatus and a method for measuring air gap by employing sonar technology. Sonar, or sound navigation ranging, is well known for use in distance measuring. For example, Duncan, U.S. Pat. No. 4,240,152 utilizes sonar for providing both distance and location information relative to a moving vehicle. Soltz U.S. Pat. No. 4,470,299 utilizes ultrasonic echo ranging to measure the level of a liquid in an open channel or other liquid container. Shelomentsev et al U.S. Pat. No. 4,452,074 utilizes ultrasonic wave propagation for monitoring the moving components of an internal combustion engine. Spectral analysis of reflected signals is employed to determine performance characteristics of the mechanism.
In accordance with the invention sound wave transmission and reception means is mounted to provide a continual measurement of air gap by transmitting an ultrasonic pulse across the air gap and receiving a reflection of the pulse. The transmission and reception means can be mounted on the stationary stator for detecting changes in air gap due to variations in the rotor, or the transmitter and receiver means can be mounted on the rotor for detecting variations in the stator.
In a preferred embodiment the transmitter and receiver comprise a single transducer element functioning as a transceiver. In this embodiment the round trip pulse propagation time is of sufficient duration to allow the transceiver to damp to its quiescent state for return of the reflected pulse.
With the transmitter and receiver means mounted on the rotor, telemetry means is employed in transmitting signals for controlling the transmitter and receiver means and for transmitting signals from the transmitter and receiver means to a gap distance determining computer.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims.